The invention concerns a flame-retardant polymer composition containing a thermoplastic polymer that contains vinyl acetate and an unsaturated elastomer as polymer components, as well as at least one flame retardant. The invention further concerns a method for producing this composition, articles made from this composition, as well as elastic composite elements having a base that has a coating consisting of this composition in at least some parts or sections of its outer surface.
Elastic elements, e.g., consisting of rubber, have no inherent flame-retardant properties as are required in some uses, in some cases pursuant to applicable law or standards. However, it is known to mix flame retardants into elastomers or (natural) rubber; however, the admixture of such additives commonly significantly reduces the elastic properties, meaning that such an element consisting of rubber cannot provide the required elastic properties in terms of desired static and dynamic properties. When used as a spring or damping element, or as a similar element, normally subject to highly dynamic stresses, e.g. in vehicles, the fire safety provisions cannot be fully met with known elastic elements.
As a result, flame-retardant coatings have come to be provided, whilst keeping the base protected by the coating free of flame retardants. Such composite elements are described, e.g., in DE 38 31 894 A1 or WO 2010/069842.
The crosslinking system is essential for the properties of an elastomer. Only the crosslinking system converts the flowable rubber into an elastomeric material that has the typical elastomeric properties, resulting in the fundamental differences from thermoplastic polymers. The type of crosslink bridge and crosslink density influence the hardness, modulus, resistance, elongation at break, tear resistance, elasticity, and the limits of its capacity to resist mechanical and thermal stresses. Additionally, polymeric compositions containing a flame retardant are known from the prior art, e.g. mixtures of ethylene vinyl acetate with ethylene-propylene-diene monomer rubber. These mixtures are either silane-crosslinked, or in most cases, crosslinked via peroxides or radiation. Such mixtures are used primarily as coatings for cables or electrical lines. Thus, for example, EP 2 343 334 A2 discloses flame-retardant compositions consisting of EVA, EPDM, and LLDPE that have a peroxide crosslinking system formed by di-cumyl peroxide. Peroxides are frequently used to crosslink rubbers. Peroxide crosslinking is the typical type of crosslink in order to crosslink rubbers without double bonds, and/or in order to obtain a particularly high crosslink density or close mesh of the crosslink, which has a positive influence on the mechanical indicators, such as the compression set, in particular at high temperatures. The often high crosslink density and the short crosslink bonds usually result in lower ultimate elongation compared to materials having the same hardness. If the surfaces of the products are no longer processed, the peroxide crosslink requires the removal of ambient oxygen during crosslinking. However, such crosslinking systems are disadvantageous for the elastic and dynamic properties, particular if large amounts of flame retardants are additionally included.
In order to obtain flame resistance, in particular under standard CEN TS 45545-2, with the related high demands on flame propagation, optical smoke density, smoke gas toxicity, and heat release rate, requires particular attention to be paid to the selection of the polymers. The smoke density and toxicity requirements, for example, rule out the use of polymers containing halogen.